1. Technical Field
The present invention relates to a liquid ejecting apparatus such as an ink jet printer and to a method for controlling the same, and more specifically, it relates to a liquid ejecting apparatus having a liquid ejecting head that activates a piezoelectric vibrator by supplying a drive signal and that thereby discharges a liquid droplet from a nozzle opening, and to a method for controlling the same.
2. Related Art
Liquid ejecting apparatuses have a liquid ejecting head capable of discharging liquid in the form of liquid droplets, and discharge various liquids from the liquid ejecting head. Image recording apparatuses such as ink jet printers are representative liquid ejecting apparatuses. Ink jet printers discharge liquid ink in the form of ink droplets onto recording paper so as to form dots, thereby performing recording. In recent years, liquid ejecting apparatuses have been applied not only to the image recording apparatuses but also various manufacturing apparatuses such as display manufacturing apparatuses.
An ink jet printer (hereinafter referred to simply as printer) will be taken as an example. This printer has a recording head and a drive signal generating circuit (drive signal generator). The recording head includes ink flow paths and pressure generators (for example, piezoelectric vibrators). Each ink flow path extends from a common ink chamber (reservoir) through a pressure chamber to a nozzle opening. Each pressure generator fluctuates the volume of the corresponding pressure chamber. The drive signal generating circuit generates a drive signal to be supplied to each piezoelectric vibrator. The drive signal supplied from the drive signal generating circuit includes a drive pulse. The drive pulse drives each piezoelectric vibrator, thereby causing pressure fluctuation in ink in the corresponding pressure chamber. By using this pressure fluctuation, an ink droplet is discharged from a nozzle opening.
In this type of printer, the size of discharged ink droplets has been reduced to meet the demand for higher image quality. Reducing the size of ink droplets reduces the diameter of dots formed on a recording medium such as recording paper, thereby improving the image resolution and reducing graininess (roughness of an image) in a low-density region. In order to reduce the size of ink droplets, the diameter of nozzle openings can be reduced. However, if the diameter of nozzle openings is reduced, the manufacturing becomes difficult, the cost therefore increases, and the accuracy easily deteriorates. In addition, clogging due to dried ink in the vicinity of a nozzle opening easily occurs. Therefore, there is a limit to the reduction of the size of nozzle openings.
To solve this problem, there is proposed an art to reduce the size of ink droplets without changing the size of nozzle openings, by devising a drive signal for driving each piezoelectric vibrator, and thereby controlling the behavior of the meniscus when an ink droplet is discharged. For example, in an ink jet recording apparatus disclosed in JP-A-2002-127418, a drive signal includes a preliminary contraction signal prior to a preparation signal. The preliminary contraction signal contracts a pressure chamber so as to push out the meniscus. The preparation signal expands the pressure chamber so as to pull in the central part of the meniscus. Ink in the minute pulled-in central part of the meniscus is discharged in the form of an extremely minute ink droplet.
However, since the meniscus is vibrated more significantly compared to the case where the preliminary contraction is not performed, the discharge of this apparatus can be unstable. For example, the trajectory of the ink droplet can curve due to the vibration of the meniscus. In this regard, the time to supply a signal (holding signal) that is generated after the preliminary contraction signal and that maintains the voltage at the end of the preliminary contraction signal can be increased so that an ink droplet is discharged after the vibration of the meniscus after the preliminary contraction is minimized. However, in that case, the increased duration of the whole waveform makes high-frequency drive difficult.